Servo controlled motor drive with a mechanical filter between the motor and the driven member



' Au 27, was I D. B. M LEOD SERVO CONTROLLED MOTOR DRIVE WITH AMECHANICAL FILTER BETWEEN THE MOTOR AND THE DRIVEN MEMBER Original FiledApril 20. 1964 Sheets-Sheet l }HEAD DRUM CONNECTIONS u n 2526}TOTAOHOMETERC|ROUIT Q s W l2 i Q I0 24 I8 21 31 3| 20 I 36- HTOCONTROLI 5 muucnoumcxcmcuns MOTOR f 33 r {5' EMA? WA *RECORDING -T0HEADS AND PLAYBACK SOURCE ClRCUlTS oms RECORD FROM HEADS R45 0 cps AALTERNATING l l c CURRENT FROM m0 maconn SUPPLY SWITCHER \44. PLAYBAQK42 CIRCUITS 39 FROMDRIVER SIGNAL musronusr g f f f REPRODUCUON f icmcuns ouwursxsmm 54 SUPPLY PULSE RECORD RECORD! SOURCE 65 PLAYBACKsenvo BINARY s cmcuns STABLE 'mvwsa TOANDFROM CONTROL lPLAYBmKMULTIVIBRATOR CONTROL TRACK WIND I men um PREAMPLIHERN 64 CONTROLOOMPLEMEN- nmv R 8% RECORD F. Toohgggn AMPLIFIER 51. FOLLQWER *FORMERAMPLIFIER ADJUSTABLE .,.na\oPosmon 0% 56 f FROM ADJUSTABLE I HEAD Wcmcun CPREMIYT 58 00mm) B.MA0LE00 flan/44 ---ro new SWITCHER cmcunsArrows/er,

Aug. 27, 1968 D. B. M LEOD SERVO CONTROLLED MOTOR DRIVE WITH AMECHANICAL FILTER BETWEEN THE MOTOR AND THE DRIVEN MEMBER 5 Sheets$heet2 ofiginal Filed April 20, 1964 EEEEKEB lwl w J INVENTOR. DONALD B.MAOLEOD M4464 ATTORNEY Aug. 27, 1968 D. B. M QLEOD 3,399,334

. SERVO CONTROLLED MOTOR DRIVE WITH A MECHANICAL FILTER BETWEEN THEMOTOR AND THE DRIVEN MEMBER Original Filed April 20, 1964 3 Sheets-Sheet5 I40x ms 145 I m v "v 600% mm M20 SQUARE WAVE DRIVE SIGNAL INVENTOR-vDONALD B. MAOLEOD BY mafia 364 ATTORNEY United States Patent SERVOCONTROLLED MOTOR DRIVE WITH A MECHANICAL FILTER BETWEEN THE MOTOR ANDTHE DRIVEN MEMBER Donald B. MacLeod, Redwood City, Calif., assignor toAmpex Corporation, Redwood City, Calif., a corporation of CaliforniaOriginal application Apr. 20, 1964, Ser. No. 360,921, now Patent No.3,342,951. Divided and this application Mar. 27, 1967, Ser. No. 649,376

9 Claims. (Cl. 318-329) ABSTRACT OF THE DISCLOSURE A low cost motor isenergized by a servo controlled high efiiciency square wave drive systemand a mechanical low pass filter element of rubber, or the like, betweenthe motor and the driven member for stable operation of the drivenmember.

This is a division of application Ser. No. 360,921, now Patent No.3,342,951, filed Apr. 20, 1964.

This invention relates to wideband recording and reproducing systems,and to methods for controlling such systems, and more particularly to aservo control system for a magnetic tape recorder using a helicalscanning head drum.

Accurate time base control is required for wide-band recording andreproducing systems because of the errors or distortions which areintroduced in the recorded or reproduced data by excessive time baseerror. If the system is used for recording a television signal, forexample, an excessive time base error may result in picture distortion,loss of synchronization and other disruptive effects. Such effects mayresult in a failure to meet established broadcast standards, but evenwhere there is no such requirement, as in aclosed circuit system, it isoften very difficult to obtain good picture stability or to reproducepreviously recorded material on a different machine. If a widebandrecording system is used for digital data, errors in the data will beintroduced unless expensive redundancy or time base compensationtechniques are used.

For these reasons, it is now conventional in wide-band recordingsystems, such as magnetic tape recording and reproducing systems, to useservo control of various parts of the system in such fashion thatsuccessively finer corrections of the time base are introduced. In themost widely used magnetic tape recording system, a rotary head drumcontaining a number of magnetic heads scans across a moving tape in agenerally transverse direction as the tape is moving at a relativelyslower speed. On reproduction of these recorded signals, the speed ofthe tape capstan, the head drum, and other mechanisms may be servocontrolled, either with respect to timing tracks laid down during theprior recording, or with respect to time stable synchronizing signals.These successive compensations may cumulatively provide highly accurateadjustment of the time base. When combined with electronicallyadjustable delay circuits, such systems can provide adjustment to withina relatively few nanoseconds of the true time base. They haveaccordingly found wide use in the recording and reproduction oftelevision signals, and in the acquisition and reproduction of digitaland instrumentation data under circumstances where extremely highinformation content must be processed.

A lower cost version of a magnetic tape recorder uses only one or twoscanning heads, and is primarily intended for use with televisionprogram material. For such applications, reductions in cost size andweight are of primary importance, although adequate time base 3,399,334Patented Aug. 27, 1968 ice stability is still required. These systemsemploy a relatively simpler arrangement in which the tape is wrappedhelically about at least a part of a split cylinder within which a headdrum rotates to pass the heads in contact with the tape. The wrap of thetape around the cylinder provides a relatively low angle of inclinationof the path of the heads relative to the longitudinal axis of the tape,so that relatively long individual tracks are defined along the tape.The simplicity of this system is partially derived from the fact thateach recorded track may be sufficient for each track to contain a fullfield of recorded picture information. Servo control of the head drumand of the capstan is utilized to insure proper recording and playback.The control is based upon use of a timing track signal recorded on thetape in accordance with head drum timing signals which represent theinstantaneous angular position of the head drum. The timing track signalaccordingly provides a reference for use during playback.

One highly desirable feature during recording is the recording of theindividual fields such that the vertical blanking interval has a desiredrelationship to the disposition of the heads relative to the tape. Thetape is so arranged, in angle and in width, that at particular times onehead is in contact with the tape at the start of a track while at thesame time the other head is in contact with the tape at the end of itstrack. Thus, there is an overlap interval in which both heads are incontact with the tape. If this overlap interval is caused to coincidewith the vertical blanking interval, or with the start of the verticalblanking interval, switching between the heads may be accomplished withminimum adverse effects. First, no transient is introduced into thereproduced video segment of the signal. Second, instantaneous timingerrors due to tension and dihedral errors at switching time causeminimum disruption if switching occurs during vertical blanking becausethe automatic frequency control circuits in a television receiver canadjust before the end of the blanking interval.

Of course, the better the time base stability of the signal on playbackafter initial recording the easier it is for the signal to meetestablished broadcast television standards, and the more readily thesystem may be used in more critical applications. The obvious expedientto provide this time base stability would be a complex servo system usedin conjunction with large and expensive motors having very low speedvariation. The use of such components, however, would materiallyincrease the complexity and cost of the system, as well as its weightand bulk. Added to this is the fact that individual motor driveamplifiers and large transformers are normally used for such systems,thus further increasing the power requirements, size, weight and theheat generated by the system. It is therefore highly desirable to beable to achieve a high degree of time base stability with low costmotors and relatively simple, lightweight driving systems.

It is therefore an object of the present invention to provide animproved control system for a magnetic tape recorder.

Another object of the present invention is to provide improved controlsystems for wideband recording and reproducing systems utilizing highspeed and low speed moving members.

Yet another object of the present invention is to provide an improvedmethod for obtaining precise control of a pair of moving elements in awideband tape recorder utilizing a scanning head drum.

A further object of the present invention is to provide a low cost drivesystem having extremely low Weight for the capstan and scanning headdrum in a recording and reproducing system.

Yet another object of the present invention is to provide an improvedmeans of controlling head drum speed and capstan speed during record andplayback modes of operation in a wideband magnetic tape recording andreproducing system.

These and other objects of the invention are met by an arangement whichprovides low cost and lightweight means for controlling and driving therelatively fast and relatively slow moving members of a head drumscanning system in a wideband recording and reproducing system. Onefeature in accordance with the invention is the use of a single servosystem which is alternately utilized to drive the head drum duringrecording and the capstan during playback. The heads are thereby causedto track properly during playback, but the introduction of cumulativetime base errors is avoided. Other features of the invention permit theutilization of relatively low cost motors, and high efliciency squarewave drive systems, together with means for filtering out short-termspeed fluctuations.

In a specific example of a magnetic tape recording and reproducingsystem in accordance with the invention, a head drum in a helical scanrecorder scans the tape along successive tracks at relatively highspeed, as the tape is longitudinally advanced at a relatively slowerspeed by a capstan. A control track representative of actual drulrnspeed variations is recorded longitudinally on the tape duringrecording. Drum tachometer signals are also compared in servo circuitsto a reference signal, and the head drum speed is stabilized inaccordance with the reference signal. During recording, the tape isdriven at a selected rate of speed directly from an alternating currentsupply, in that the frequency of the signals from the source directlycontrols the speed of the tape capstan. On playback, however, thereproduced control signal is compared to the head drum speed signal, andused to control the capstan instead of the head drum, which at this timeis controlled directly by the frequency of the signal from thealternating current supply. Although the tape is advanced such that theheads properly scan the recorded tracks, this arrangement greatlyimproves the time base stability of the system, because capstan speedvariations on playback are not directly translated as drum speedvariations. Instead, the stability of the supply is relied on as theprimary factor in holding the recovered video signal constant, and anychanges in capstan speed are very small compared tothe much faster headdrum speed.

Examples of specific drive amplifier circuits in accordance with theinvention provide superior combinations of low cost, low weight and highefliciency for high performance systems. Semiconductor switchingelements coupled -in circuit with the motor are used to provide a highefficiency square wave drive signal through a relatively simpleswitching control using -a minimum of transformer components. Inconjunction with this system, odd harmonic components in the drivewaveform, as well as torque pulsations in the operation of the motoritself, do not introduce substantial speed variations in the drivenmember because a low pass mechanical filter is used in the coupling soas to eliminate these speed variations. One aspect of these arrangementsis the use of mutually coupled transformers in such fashion as toprovide proper commutation of the switching device. Another aspect isthe use of starting and overload control circuits which prevent adverseeffects from transients which may result in short circuit conditions orthe failure of the circuit to start on initial application of power.

A better understanding of the invention may be had by reference to thefollowing description, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a combined simplified perspective and block diagramrepresentation of one system in accordance with the invention, showingservo circuits and drive arrangements in accordance with the invention;

FIG. 2 is a schematic diagram of square wave generator circuits and oneform of motor drive circuit in accordance with the invention;

FIG. 3 is a schematic diagram of another form of motor drive circuit inaccordance with the invention; and

FIG. 4 is a perspective view showing in greater detail a compliantcoupling employed in the arrangement of FIG. 1.

A wideband recording and reproducing system in accordance with theinvention is illustrated in general form in FIG. 1, with many of thesignal processing circuits and tape handling mechanisms being omittedfor simplicity because they may be conventional. A tape 10 is fedbetween a supply reel 11 and a takeup reel 12 past an upstanding splitcylinder 14 about which the tape 10 is wrapped helically for a selectedcircumferential distance, which here defines a wrap angle slightly inexcess of The cylinder 14, greatly simplified inform, has a centralcircumferential slot within which rotates a head drum 15 containing apair of magnetic heads 17, 18 which have pole surfaces at the outerperiphery of the head drum 15. The heads 17, 18 pass in contact with theslowly moving tape 10 disposed about the cylinder 14, and scan alongsuccessive magnetic tracks on the tape 10 which are at a relativelyslight angle (e.g. 9) relative to the longitudinal axis of the tape. Aring and brush assembly (not shown in detail) may be utilized forapplying sig nals to be recorded to the heads, and for coupling offreproduced signals from the heads 17, 18. The tape is longitudinallyadvanced at a relatively low speed such as 3.75" per second under thecontrol of a capstan 20- and pinch roller 21, the capstan being drivenby a hysteresis synchronous motor 22 as described below. The shaftcoupling between the capstan 20 and the motor 22 includes a compliantcoupling in the form of a ball or disk 23.

The head drum 15 also includes a set of integral magnetic elements 24,25 and 26 disposed in the surface of the head drum 15, andcircumferentially placed so that one magnetic element 24 is 180displaced from another element 25, and the third element 26 is displacedby only a relatively small angle, such as 15 from the second element 25.A magnetic head 28 positioned adjacent the head drum 15 generatessignals as the magnetic elements 24-26 pass adjacently, these signalsbeing coupled to the tachometer circuit described below. The pairedmagnetic elements 25, 26 not only provide an indication of the angularposition of the head drum 15, but also enable a selected index positionon the head drum 15 to be distinguished from the position which is 180displaced. The system also includes conventional erase head means (notshown), an audio head 30 and a control track head 31 disposed adjacentthe opposite edges of the tape 10. The audio head 30 is coupled to audiorecording and playback circuits 33, and the control track head 31 iscoupled to control track circuits described below.

The head drum 15 is driven from an induction motor 35 by a couplingthrough a shaft 36 which also includes a compliant coupling in the formof a rubber ball or disk 37. Energizing and speed determining signalsfor the drum motor 35 and the capstan motor 22 are derived separatelyfrom a motor drive amplifier 39 which itself is powered from a DC supply40, shown separately for clarity only, and from an alternating currentsupply 42. The source used for driving a given motor depends upon themode of system operation. In the record mode a two-pole, twopositionswitch 44 couples the motor drive amplifier 39 to the drum motor 35,while a two-pole, two-position switch 45 couples the AC supply 42 to thecapstan motor 22. In the playback mode the switches 44, 45 couple themotor drive amplifier 39 to the capstan motor 22, and the AC supply 42to the drum motor 35, respectively. Both the motors 22, 35 aresynchronous, in the sense that they rotate at a speed which is directlydetermined by the frequency of the alternating drive signal. Therotational rate may be a multiple or sub-multiple of, as well as equalto, the drive signal frequency.

Timing pulses to control the switching of the motor drive amplifier 39are derived from servo circuits, as described below: In this arrangementonly one motor drive amplifier is used, with consequent savings of size,weight and expense. In the event that separate amplifiers are needed fora specific purpose the alternating current signal can still provide atiming reference. For example, timing pulses may be generated from theAC wave form by a zero crossing detector which actuates a monostablemultivibrator at each positive (or negative) going zero crossing of thesinusoidal wave, or by a high-gain, current-limiting amplifier whichserves as a square wave generator.

The recording and reproducing circuits for this system have not beenshown in detail, but are only represented in generalized form. Signalsto be recorded are provided from a data source 50 through recordingcircuits 51 to the heads 17, 18 on the head drum 15. Reproduced signalsgenerated during the playback mode are coupled from the heads 17, 18through head switcher circuits 53, where they are combined under thecontrol of switching signals derived from the tachometer circuit into asingle, substantially continuous television signal. This signal isthereafter further amplified and processed in signal reproductioncircuits 54, and provided as output signals from the system.

The remainder of the control system in accordance with the invention hasbeen illustrated only in generalized form. Signals from the tachometerhead are coupled to a tachometer circuit 56 which generates a squarewave signal representative of the instantaneous angular position of thehead drum, and also the phase of the head drum relative to the tape.This signal is applied to the head switcher circuits 53, and alsocoupled through either of two delay circuits, an adjustable headposition delay circuit 57 and an adjustable tracking delay circuit 58.In the record mode, a two-position, single-pole switch 60 couples thetiming signal to servo circuits 61 through the head position delaycircuit 57, whereas in the playback mode, the switch couples the signalsthrough the tracking delay circuit 58. The head position delay circuit57 introduces a relatively small control range, in order to permit headswitching to be adjusted to compensate for circuit drifts and likeeffects. The tracking delay circuit 58 provides a substantially greaterdelay, equal to a full wavelength at the operating frequencies of thetiming signal. This full range of tracking delay permits either head toscan given alternate tracks on the tape, for best matching of playbackcharacteristics and minimization of instantaneous time base errors atswitching times.

When operating in the record mode, the square wave timing signals fromthe tachometer circuit 56 are applied through a control track-recordamplifier 63 to the control track head 31. The square wave recordedsignal is later reproduced as a series of pulses, due to theresponsiveness of the conventional head 31 to rate of change of flux.The pulses are applied through a control track preamplifier 64 to aswitch 65 and thence to the servo circuits 61 as a reference signal forcomparison to the timing signals from the tachometer circuit 56. Thisinput of the servo circuits 61 may be disabled by a switch 66 which iscoupled to ground when the system is in the stop or rewind modes.Alternatively, in the record mode, the switch 65 provides a referencesignal to the servo circuits 61 from a vertical synchronizing pulsesource 68, or some other source of time stable reference pulses. In thetypical situation, a vertical sync pulse source 68 will comprise astudio video signal source, to which a synchronizing signal strippercircuit is coupled to derive the 60 cycle per second vertical syncpulses.

In both the record and playback modes, the servo circuits 61 compare thephase of the reference signals to those of the reproduced timingsignals, and generate a varying amplitude signal which is representativeof the phase relation. The output signal is converted into anappropriate signal for controlling the switching of the motor driveamplifiers, by application to :a pulse generator system driven by acontrolled oscillatory circuit. Here a frequency controllable cycle persecond astable multivibrator 70 is coupled to a binary. divider 71, suchas a bistable multivibrator circuit which receives trigger input signalsfrom the astable multivibrator 70. The square wave output signal fromthe binary divider 71 is a nominal 60 cycle per second wave which drivesa driver transformer circuit 74 through a complementary emitter followercircuit 72. The output signal from the driver transformer circuit 74 istherefore an alternating polarity square wave suitable for switching thestates of the motor drive amplifiers in alternating fashion.

The operation of the system of FIG. 1 provides a substantial improvementin the time base stability of wideband recording and reproducing systems:of this type, with actual simplification of the drive systems which areused. As described immediately above, the error signal from servocircuits 61 is used to provide a square wave whose instantaneousfrequency is representative of a desired speed correction. This squarewave switches the motor drive amplifier 39 through the drivertransformer 74 at a corresponding rate, nominally 60 cycles per second.In accordance with the conventional operation of an induction motor 35 asignal of this frequency drives the output shaft 36 and the associateddriven member at a speed of 30 cycles per second. With a pair ofmagnetic heads 17, 18 on the drum 15, sixty tracks per second arerecorded along the tape 10, there being one track per field. Because thevertical sync pulses from the source 68 are compared in phase relationto the timing waveform from the tachometer 56, and because this timingwaveform is controlled by the position of the head drum 15 itself, thevertical blanking interval in the recorded television signal ismaintained such that it falls within the overlap interval during whichboth heads 17, 18 are in contact with the tape 10. Stated in anotherway, the magnetic elements 24-26 may be so positioned relative to theheads 17, 18, that the signals which are generated from the magneticelements 2426 insure, when the sys-' tem is properly operating, that theservo control holds the blanking interval in proper relation to theoverlap interval. Because the head switcher circuits 53 are alsocontrolled by the timing waveform from the tachometer circuit 56, thedisruptive effects of introducing a switching transient in the videoportion of the picture are avoided, and time base errors are alsominimized.

It will be noted that the square wave from the motor drive amplifier 39which powers the induction motor 35 in the record mode contains oddharmonic frequency components which inherently present torquefluctuations and consequent speed variations in the operation of theinduction motor 35. In addition, although induction motors are ofrelatively low cost, they have inherent speed fluctuations, even with anideal input wave. Through the use of the compliant coupling 37, however,in conjunction with the inertia of the motor 35, shaft 36 and head drum15 system, these irregularities in the rotational speed of the head drum15 are substantially eliminated, The speed of the head drum 15 isaccordingly servoed to the time stable sync pulses during recording, andthe head drum 15 rotates with only the inherent time base displacements,principally flutter, which cannot be completely eliminated in anymechanical system.

The time base errors in the timing waveform which is recorded on thecontrol track, however, also contain a component which is derived frominstantaneous speed variations in the operation of the capstanmechanism. However, in recording the capstan motor drive amplifier 44 isoperated directly from the 60 cycle alternating current supply 42 andthe peak-to-peak time stability of this signal is excellent.Accordingly, the principal variant from a true 60' c.p.s. wave in therecorded timing waveform is the component derived from the head drumspeed variation.

On playback, the drive sources for the drum motor 35 and the capstanmotor 22 are reversed, so that the scanning speed of the head drum isnow determined solely by the peak-to-peak stability of the signal fromthe alternating current supply 42. The tracking of the heads 17, 18 onthe previously recorded patterns is therefore determined solely byvariations in the speed :of the capstan 20, as determined by theoperation of the capstan motor 22. It is found that this system providesan appreciable increase in the stability of the video signal onplayback. Despite the use of low cost motors and square wave drives, thevideo playback stability is held, in one typical practicalexemplification of the invention, to within +0.15 microsecond of thetrue time base for one drum revolution.

Consideration of the various factors which contribute to proper systemoperation and to time base errors will permit an appreciation of theseconsiderations. The head to tape speed is very high (approximately 650i.p.s.) relative to the longitudinal tape speed (3.75 i.p.s.). The servois referenced to vertical sync, and maintains the switching time withinor at the start of the vertical blanking interval. The capstan 20, beingdriven directly by the alternating current supply 42, does not affectoperation of the servo and has very slight effect on the headto-tapespeed. The stability of the supply 42 insures against major deviations,but longitudinal tape speed variations are not of substantial effectbecause of the much greater head drum speed. As the head drum scans afull track at 650 i.p.s., the tape is moved a very much smaller distanceat 3.75 i.p.s.

When the head drum 15 is servoed during both record and playback, theflutter on playback is the cumulative total of head drum and capstan(longitudinal tape speed) flutter during record plus head drum andcapstan flutter during play-back. The capstan variations may result fromtape tension variations which cause slippage, :or from eccentricities inthe capstan or its drive shaft. On playback, however, capstan fiutterbecomes significant because capstan speed variations affect thedisplacement of the control track to which the head drum is referenced.To compensate for these displacements the head drum must be speeded upor slowed down proportionately. Thus a 1% change at 3.75 i.p.s. nowrequires a compensating 1% change at 650 i.p.s. in order for the head tostay precisely on track. Consequently, appreciable time base variationscan be introduced in the signal being reproduced.

When the capstan is servoed and the head drum is driven directly fromthe AC supply, however, the reproduced video signal is subjected only tothe flutter inherent in the head drum over a scanned track or onetelevision field. All that the capstan system need do is maintain thetrack being scanned under the magnetic head which is then reproducingsignals. Any changes in speed needed to maintain this condition are atthe most small fractions of the scanning speed. Therefore the primarystability of the recovered signal is determined by the drum, and theeffects of capstan speed variations are second order in magnitude.Additionally, playback in this manner permits locking of the head drumto the most stable reference available and avoids the time displacementerrors present in the control track. These errors will typically be ofthe order :of 250 microseconds, peak-to-peak, at 2 c.p.s. in a practicalsystem. The time base stability of the reproduced video accordingly isgreatly enhanced.

Additional improvements in performance are achieved by this arrangement.The tracking controls 57, 58 may be adjusted without affecting thereproduced picture, because the head drum is driven independently.Change of the control settings alters capstan operation only, so thatthere is no appreciable video sync frequency shift, and no appreciableraster wobble. This is of particular significance at start of playback,because the system can lock on to the proper track relationship withminimum raster wobble.

It is found that the servo system considerations for the head drum,which operates at much higher speed, are all that need be considered inthe use of a system of this nature, Satisfaction of the servorequirements of the head drum automatically satisfy the requirements ofthe cap stan drive system.

A particularly useful system for generating the triggering pulses forthe motor drive amplifiers, and for providing the motor drive amplifierfor the head drum, are shown in the schematic diagram of FIG. 2.

In FIG. 2, input signals to the astable multivibrator 70 of FIG. 1 areprovided from the servo circuits 61 of FIG. 1 in the form of an analogor varying amplitude direct current level. This signal, in one practicalsystem, is caused to vary about a nominal +6 volt level, and isconverted to a nominal zero volt level by a Zener diode 80. Plus orminus variations of this signal from the zero volt level determine thedeviation in frequency, from a nominal frequency of 120 c.p.s., of anastable multivibrator 70 which comprises principally a pair oftransistors 81, 82 having a timing network 85, 86 and the conventionalcross-coupling between them. The opposite states of the astablemultivibrator 70 are made highly asymmetric, in order to increase thefrequency stability of the system. The positive-going pulses at thecollector of the transistor 82 are brief in duration, relative to thenegative-going portion of the wave, the positive-going portions of thewave being determined by the conducting interval of the transistor 82.With this arrangement, a resistor 85 and a capacitor 86 in the timingcircuit are the principal frequency determining factors of the circuit.

The positive-going pulses from the multivibrator 70 are applied astrigger impulses to the two halves of a binary divider 71, reversing thestate of the binary divider 71 at the nominal c.p.s. rate, thusproviding a square wave having a nominal 60 c.p.s. rate. The binarydivider comprises primarily a pair of transistors 90, 91, the collectorof the second transistor 91 providing a system output signal. Theimpedance level of this output signal is lowered by the associatedcomplementary emitter follower 72, and the output signals are thenapplied to the driver transformer 74 which is coupled to the motor driveamplifier 39 for the drum motor 35.

The pulse waveforms applied to the primary 94 of the transformer areconverted into control signals of appropriate polarity for four separatetransistors 96, 97, 98 and 99 coupled together in a bridge configurationin the motor drive amplifier 39. The bridge is coupled across a DCsupply in the form of 18 volt and +12 volt sources. Secondary windingsin opposite senses are coupled in the base-emitter circuits of eachtransistor of a different pair in the bridge, such as the pair 96, 97and the pair 98, 99. A square wave output voltage is taken from themidpoint between each pair of transistors 96, 97 or 98, 99. At anyinstant, two diagonally opposed transistors 96 and 99 or 97 and 98 willbe switched on, providing a potential difference across the windings ofthe drum motor 35, which potential difference is alternated in polaritywith the square wave drive. The bases of two transistors, e.g.transistors 96 and 99, will be driven positive concurrently, causing thetransistors 96, 99 to conduct and completing a current path between the18 volt source, the transistor 96, the motor windings, the othertransistor 99 and the +12 volt source. When the opposite transistors 97,98 are switched on, the applied DC voltage is of the opposite polarity.Because the full potential difference between the supply terminals (inthis case 30 volts) is applied to the motor 35 for each half cycle, thepeak-topeak or effective applied square wave amplitude is twice themaximum level available from the supply by virtue of the polarityreversing action of the motor drive circuit.

Therefore, a 60 cycle square wave for operating the motor with highefficiency is obtained from a circuit constructed of componentscontaining relatively little iron (the only iron being that in thedriver transformer). The odd harmonic components of the square wavedrive signal do not result in instantaneous drum speed variations ofappreciable magnitude, because of the use of a compliant coupling,described below, which forms a low pass mechanical filter in conjunctionwith the inertia of the head drum system.

The motor control circuits include a start circuit for increasing thestarting torque so as to provide superior acceleration to normal runningspeed. Under the high current starting conditions, a motor start relayis energized, closing a switch 102 and placing a current limitingresistor 104 and a start capacitor 105 in parallel with the normalrunning capacitor 107. The motor start windings 109 are thereforeutilized, until the drive current drops, at which time the motor startrelay 101 is de-energized, the start capacitor 105 is removed from thecircuit. A thermal switch 110 opens to disconnect the windings, in theevent of overheating of the motor 35.

The low voltage DC supply square wave drive arrange ment, as described,provides merely one example of a circuit which may be utilized inaccordance with the invention. A preferred circuit is shown in FIG. 3,in which higher voltage but relatively lower cost silicon-controlledrectifiers are employed to generate the square wave drive signal. In thesystem of FIG. 3, relatively high voltage input signals (eg 117 volts at60 cycles per second) are applied across a full wave diode rectifier113, across the output terminals of which is coupled a ripple filter114. The DC signal of approximately 150 volts, which is therebygenerated, is applied across a bridge comprising four silicon-controlledrectifiers 116, 117, 118 and 119. The output terminals of thesilicon-controlled rectifier bridge are coupled to the capstan motor 22,or the head drum motor as desired, or in accordance with the inventionthe square wave drive signal may be switched between these motorsdepending upon the mode of operation. Only the capstan motor 22 has beenshown for simplicity.

The silicon-controlled rectifier 'bridge operates as an inverter, underthe control of square wave drive signals provided from an associatedsource to the primary 121 of a driver transformer 120, the separatesecondary windings 122, 123, 124 and 125 of which are coupled to thecontrol electrode and cathode circuits of the individualsilicon-controlled rectifiers 116-119 respectively. Thesilicon-controlled rectifier bridge also includes reactive components inthe output circuit coupling, in the form of mutually coupled inductances127, 128 which are connected across resistances 130, 131 at the outputterminals of the silicon-controlled rectifier bridge. Biasing diodes135, 136, 137 and 138 are coupled within the bridge to shunt thesilicon-controlled rectifiers 116-119.

The circuit also includes a protection circuit 140 comprising a relaycoil 141 controlling an armature 142, a resistor 144 in series with therelay coil 141, and a resistor 145 and capacitor 146 which shunt therelay coil 141. The protection circuit 140 prevents the application ofexcessive current to the silicon-controlled rectifiers in the event of ashort circuit, or incorrect commutation of the silicon-controlledrectifiers, as described above.

In the operation of the arrangement of FIG. 3, the full wave rectified,150 volt DC signal applied across the silicon-controlled rectifier'bridge is inverted under control of the square wave drive signal at thetransformer 120 at a rate which provides the varying frequency squarewave drive signal desired for the motor 22. The polarities of thesecondaries 122-125 which control the firing of the individualsilicon-controlled rectifiers 116-119 are arranged to control conductionin opposite pairs 116 and 119 or 117 and 118. Thus the polarity of thevoltage applied across the motor is reversed at a controlled rate, andthe motor 22 and associated elements are driven synchronously therewith.The only inductive elements which are used are the driver transformerand the mutually coupled inductances 127, 128, which are small in size,and no power transformer with attendant weight and size and heat isrequired. In addition, the square wave switching provides extremely highefficiency in driving the motor in an economical fashion.

Because silicon-controlled rectifiers remain on, once fired, unless theanode goes negative relative to the cathode, some simple means ofcommutating the action of the silicon-controlled rectifiers in thebridge, to insure positive extinction of previously fired rectifierswhen the remaining rectifiers are fired, is highly desirable. This isachieved, in accordance with the present invention, by the mutuallycoupled inductances 127, 128, in association with the capacitor 129which bridges the motor. When a silicon-controlled rectifier, such asthe rectifier 117, has remained conducting for its half cycle, and afiring signal is received for the coupled rectifier 116, a firing signalis also received for the rectifier 119. The resulting current surgingthrough the mutually coupled inductances 127, 128 drives the anodes ofthe previously conducting rectifiers 117, 118 negative, thusextinguishing the previously conducting rectifiers 117, 118 andproviding the desired commutating action. In addition, the provision ofthe mutually coupled inductances 127, 128 in the depicted circuitdevelops a limiting effect on the rise times of the voltages applied tothe silicon-controlled rectifiers 116- 119, thus avoiding spontaneousturn-on from junction capacitance effects. The voltage drop across thediodes -138 aids in insuring turnoff.

The protection circuit operates in a cycling fashion to interrupt thepower signal to the silicon-controlled rectifier bridge in the event ofcircuit failure or improper firing. On startup, for example, transientconditions when power is initially applied may turn on all therectifiers, because of the fact that firing of one pair is used toassist extinction of the other pair. If the rectifiers 116-119 areimproperly fired, so that a short circuit effectively exists, then thevoltage drop across the resistor increases, and the relay coil 141 opensthe single pole, single throw armature 142, interrupting the power tothe silicon-controlled rectifiers 116-119. After a 'brief time interval,determined by the passive circuit comprising a resistor 144 and timedelay capacitor 146, the relay coil 141 is deenergized, releasing thearmature 142, so that power is again applied to the silicon-controlledrectifier circuit. The protection circuit 140 operates if thesilicon-controlled rectifiers 116-119 fail to start when power isinitially applied, or if a transient signal disturbs the regularcommutation sequence. By interrupting the power signal in this manner,all silicon-controlled rectifiers are extinguished, and only the properrectifiers are fired on the application of the next input drive signalwhen the protection circuit 140 again closes.

In the event that a short circuit condition is maintained, theprotection circuit 140 also operates, but here the operation is for thepurpose of interrupting the application of power at a relatively lowfrequency to limit the average power applied to the load. The cyclingfrequency of the protection circuit 140 is such that the resistor 145can readily absorb the power applied under the given circumstances.

A detailed example of one form of compliant coupling which may be usedin accordance with the invention in conjunction with square wave drivesystems is shown in FIG. 4, as employed with a head drum 15. Aparticular shaft 36 which couples the motor (not shown in FIG. 4) to thehead drum 15 is a two piece section which is coupled together lby aresilient symmetrical rubber member 148. With a low cost drive motor,such as an induction motor which inherently has torque fluctuations, theapplication of a square wave drive signal, which contains odd harmonicfrequency components, introduces further instantaneous speed variationsin the speed of the driven 1 1 member. In accordance with an aspect ofthe invention, this mechanical system forms a mechanical low passfilter. The rubber disk 148 is selected to have a torsional moment whichis dependent primarily upon the moment of inertia of the shaft 36 andthe head drum 15. In one specific example, the head drum had a 6.75 inchdiameter, a moment of inertia of 0.00445 slug-ft}, and a nominalrotation rate of 1800 r.p.m. A synthetic rubber coupling member wasemployed in the drive. The coupling member was a unit sold by the LordMfg. Co., Erie, Pennsylvania, under Catalog Number V-1211-1-189, and hada 1.7 in.-lb. torque specification. Such couplings are mtended for usefor adjustment of alignment of driving and driven members, but whenappropriately selected may be used in systems in accordance with theinvention.

Although there have been described above and illustrated in the drawingsvarious systems and methods for wideband recorders and reproducers, andoutput drive systems for the operation of scanning and rotation memberstherein, it will be appreciated that the invention is not limitedthereto. It may not be required, for example, to economize by utilizinga single motor drive amplifier, or it may be preferred to operatedifferent motor drive amplifiers from a single DC supply, or from DCsupplies at different voltage levels. In this event, the alternatingcurrents supplied may still be utilized for timing purposes, inasmuch asthe zero crossing detector or other form of square Wave generatoroperating off the alternating current signal may be used to effect theswitching control of the motor drive amplifier when control is notexercised by the servo system.

It will therefore be appreciated that these and other modifications maybe made in accordance with the spirit of the invention, and it should beunderstood that the invention is to be defined by the appended claims.

What is claimed is:

1. In a system for driving a member at controlled speed dependent uponan error signal, the combination of: means responsive to the errorsignal for generating a square wave switching signal; bridge invertermeans coupled to be controlled by the square wave switching signal;synchronous motor means driven by the bridge inverter means; andmechanical low pass filter means coupling the motor means to the memberto be driven.

2. In a system for rotating a driven member under control of a servoerror signal, the combination comprising: a variable frequencyoscillator having a selected nominal frequency and coupled to vary infrequency in accordance with the servo error signal; a square wave powersignal generating circuit responsive to the signal from the oscillator;a motor coupled to be driven by the square wave power signal; andmechanical low pass filter means coupling the motor to the member to bedriven.

3. A low cost system for rotating a driven member with variable butcontrolled speeds corresponding to a servo error signal, including thecombination of: variable frequency oscillator means having a controlinput terminal and a selected nominal output signal frequency, thecontrol input terminal being coupled to receive the signal error signaland the nominal frequency being a multiple of the nominal rate ofrotation of the driven member; binary divider means coupled to receivethe signal from the variable frequency oscillator means and to generatea square wave signal of half frequency; a synchronous motor, a DC supplysemiconductor bridge means including a plurality of semiconductorswitching elements coupled to provide power signals of opposite polarityto the synchronous motor; firing circuit means coupled to receive thehalf frequency signal and to control the semiconductor switchingelements of the semiconductor bridge means, the firing circuit meansincluding means responsive to the square wave signal and transformercoupled to the semiconductor switching elements; and means including aresilient rubber element connecting the synchronous motor to the memberto be driven.

4. A system for rotating a driven member simply and economically with alow instantaneous speed variation comprising the combination of a DCsupply means, switching inverter means coupled to the DC supply means,means coupled to control the switching inverter means at a selectedrate, to provide a square wave power signal having a controlledperiodicity, a motor coupled to receive the square wave signal and beingof the type to be operated synchronously therewith, and means includinga compliant coupling providing a mechanical low pass loader coupling themotor to the driven member, in conjunction with the inertia of thedriven member.

5. In a system for rotating a driven member under control of a servoerror signal, the combination comprising: a

variable frequency oscillator having a selected nominal frequency andconnected to vary in frequency in accordance with the servo errorsignal; a silicon-controlled rectifier circuit responsive to the signalfrom the variable frequency oscillator and generating a square wavepower signal; an induction motor coupled to be driven by the square wavepower signal; and a mechanical low pass filter means coupling connectingthe induction motor to the member to be driven.

6. A low cost system for rotating a driven member with variable butcontrolled speeds corresponding to a servo error signal, including thecombination of: variable frequency oscillator means having a controlinput terminal coupled to receive the servo error signal and providing asignal at a nominal frequency which is double the nominal rate ofrotation of the driven member; binary divider means coupled to receivethe signal from the variable frequency oscillator means and to generatea square wave signal of half frequency; silicon-controlled rectifiermeans coupled to receive the square wave signal and to provide a squarewave power signal at the same frequency, synchronous motor means coupledto receive the square wave power signal and to be driven synchronouslythereby; and a rubber coupling means connecting the induction motor tothe member to be driven.

7. In conjunction with a tape drive mechanism, an electrical circuit fordeveloping substantially square wave signals of alternating polarity forapplication to a drive motor from a unidirectional voltage supply insynchronism with applied control signals comprising foursilicon-controlled rectifiers coupled in a bridge configuration, aunidirectional voltage supply coupled across a first pair of opposedterminals of the bridge configuration, a drive motor coupled across asecond pair of opposed terminals of the bridge configuration, means forinitiating conduction concurrently in oppositely situatedsilicon-controlled rectifiers of said bridge by pairs for respectiveopposite polarities of applied control signals including a transformerhaving a primary winding for receiving control signals and a pluralityof secondary windings coupled respectively to individualsilicon-controlled rectifiers to initiate conduction therein, and meansfor extinguishing conduction in said silicon-controlled rectifierscomprising a plurality of mutually coupled inductors connected to applya reverse polarity voltage across a conducting pair ofsilicon-controlled rectifiers when the remaining pair ofsilicon-controlled rectifiers is triggered to conduction.

8. An electrical circuit in accordance with claim 7 above furtherincluding means coupling said voltage supply across said bridgecomprising a relay Winding coupled to receive at least a portion of thecurrent supplied to the bridge, a pair of normally closed relay contactsresponsive to said winding and connected in series in a current pathbetween the voltage supply and the bridge, and means for maintainingsaid relay winding energized for a predetermined time interval followingthe opening of said relay contacts at a selected level of current insaid winding in order to protect said circuit from a current overload.

9. In conjunction with a tape drive mechanism, an electrical circuit fordeveloping square wave signals of alternating polarity for applicationto a drive motor from a unidirectional voltage supply in synchronismwith applied control signals comprising four transistors coupled in a 13bridge configuration, a unidirectional voltage supply coupled across afirst pair of opposed terminals of the transistor bridge configuration,a motor circuit coupled across a second pair of opposed terminals of thetransistor bridge configuration, and a transformer comprising a primaryWinding for receiving control signals and a plurality of secondarywindings coupled to control conduction in the transistors by oppositepairs in response to said control signals, the motor circuit including amain winding and a starting winding, a' capacitor in series with thestart winding, a relay winding in series with the main winding and apair of relay contacts coupled to switch additional capacitance inparallel with said capacitor when current through the main winding andthe relay winding exceeds a predetermined level.

References Cited UNITED STATES PATENTS 3,171,075 2/1965 Kirk 3l8-3413,214,666 10/1965 Clerc 318-341 X 3,223,912 12/ 1965 Sheheen 318341 1ORIS L. RADER, Primary Examiner.

J. J. BAKER, Assistant Examiner.

